Macroscopic Quantum Tunneling and Coherence

Does Quantum Mechanics break down for big (macroscopic)-objects? that is: can an object large on the scale of atoms be found in a quantum superposition? Most physicists probably think the theory will still hold up, but that has important philosophical consequences. In the last decade, Tony Leggett and collaborators have written a lot about this, and have stimulated a number of interesting experiments on SQUIDS. These small superconducting devices support currents in a loop structure, the idea being that the current flow can be in a superposition of clockwise and anti-clockwise directions. This they call "Macroscopic Quantum Coherence (MQC)", a fascinating phenomenon not yet convincingly seen in experiments. A related effect is the tunnelling of current from a metastable state in SQUIDS. In contrast to MQC, this "Macroscopic Quantum Tunnelling (MCT)" has been experimentally observed.
See A.J. Leggett in Quantum Tunneling in Condensed Media, eds. Yu Kagan and A.J. Leggett, (Elsevier Science 1992) for a recent discussion of the theory, and M.H. Devoret in the same volume for a discussion of the experiments.
Last modified: May 31, 1994

Ard Louis, ardlouis@lassp.cornell.edu